Communication system, communication control method, and transmission apparatus

ABSTRACT

A communication system includes a source node and an end node of a first working path, a source node and an end node of a second working path, and a source node and an end node of a common path serving as a protection path shared by the first and second working paths. The source node of the common path transfers one of signals received from the source nodes of the first and second working paths to the common path, and the end node of the common path transmits the signal received through the common path to each of the end nodes of the first and second working paths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2013-119003, filed on Jun. 5,2013, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a communication system, acommunication control method, and a transmission apparatus.

BACKGROUND

In recent years, an optical transport network (OTN) is beingstandardized in International Telecommunication Union-Telecommunicationsector (ITU-T) and Institute of Electrical and Electronic Engineers(IEEE).

The OTN can transmit different kinds of client signals in a transparentmanner in an optical network to which a wavelength division multiplexing(WDM) technique is applied.

As a method that is high in reliability on a failure and capable ofeffectively using network resources in the OTN, there is a sharedrestoration scheme (for example, see WO2004/075494).

In the shared restoration scheme, a band of a protection path placed ina route completely different from a working path is secured in advance,and a band of one protection path is shared by a plurality of workingpaths. When a failure occurs in the working path, a control signal suchas an automatic protection switching (APS) signal is transmitted in theprotection path to secure a physical connection of the protection pathand perform switching from the working path to the protection path.

FIG. 37 illustrating an exemplary network configuration in the sharedrestoration scheme. For example, a network illustrated in FIG. 37includes communication nodes #1 to #11, and the communication nodes #1to #11 are connected in a mesh form, for example.

A working path A is set to a route passing through the communicationnodes #1, #2, #3, and #4. Further, a working path B is set to a routepassing through the communication nodes #8, #10, #11, and #9. For eachof the working path A and the working path B, a physical connectionsetting is secured in the corresponding communication node #j (j=one of1 to 11).

Meanwhile, a protection path A for the working path A is set to a routepassing through the communication nodes #1, #5, #6, #7, and #4. Further,a protection path B for the working path B is set to a route passingthrough the communication nodes #8, #5, #6, #7, and #9.

The protection paths A and B are in a state in which a physicalconnection setting has not been secured in the correspondingcommunication node #j but a band has been reserved. Further, between thecommunication node #5 and the communication node #7, reserved bands ofthe protection paths A and B are shared by the working path A and theworking path B.

When a failure occurs in the working path A (or the working path B),path switching is performed so that a signal (traffic) which has beentransmitted through the working path A (or the working path B) isavailable to be transmitted through the protection path A (or theprotection path B).

Here, WO2004/075494 discloses a method of switching a path quickly inthe shared restoration scheme. FIG. 38 illustrates an exemplary networkconfiguration before a failure occurs, and FIG. 39 illustrates anexemplary network configuration when a single failure occurs.

As illustrated in FIG. 38, a physical connection setting is securedbefore a failure occurs in each of the communication nodes #1, #5, #6,#7, and #4 positioned in the route through which the protection path Apasses. Meanwhile, in each of the communication nodes #8, #5, #6, #7,and #9 positioned in the route through which the protection path Bpasses, a physical connection setting is not secured, and a protectionpath band is reserved.

Further, a failure is assumed to occur in the working path A (forexample, between the communication nodes #2 and #3) as illustrated inFIG. 39. In this case, since physical connection is secured between thecommunication nodes #1 and #4 respectively corresponding to a sourcenode and an end node of the protection path A, by performing an APSswitching process in the end node #4, fast path switching from theworking path A to the protection path A can be performed.

However, when a failure occurs in the working path B, a physicalconnection setting is not secured in the protection path B. Thus, it isnecessary to transmit a control signal such as an APS signal from thecommunication node #8 to the communication node #9 through thecommunication nodes #5 to #7 to secure the physical connection settingof the protection path B. After the physical connection setting of theprotection path B is secured, switching the working path B to theprotection path B is performed.

As described above, in the shared restoration scheme, among a pluralityof working paths sharing the protection path, only one working path cansecure the physical connection in the protection path in advance. Thus,when a failure occurs in the working path that does not secure thephysical connection in the protection path in advance, since it isnecessary to transmit a control signal after detecting the failure, andthus it is unavailable to switch the working path to the protection pathquickly.

SUMMARY

An aspect of a communication system includes a source node and an endnode of a first working path, a source node and an end node of a secondworking path, and a source node and an end node of a common path servingas a protection path shared by the first and second working paths,wherein the source node of the common path transfers one of signalsreceived from the source nodes of the first and second working paths tothe common path, and the end node of the common path transmits thesignal received through the common path to each of the end nodes of thefirst and second working paths.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an exemplary configuration of acommunication network according to an embodiment;

FIG. 2 is a diagram illustrating an exemplary hardware configuration ofa communication node illustrated in FIG. 1;

FIG. 3 is a functional block diagram illustrating a communication nodeillustrated in FIG. 1;

FIG. 4 is a diagram illustrating an exemplary format of an ODUk framecompatible to the ITU-T recommendation G.709;

FIG. 5 is a diagram schematically illustrating a cross connect settingin a communication node #1 illustrated in FIG. 1;

FIG. 6 is a diagram schematically illustrating a cross connect settingin a communication node #8 illustrated in FIG. 1;

FIG. 7 is a diagram schematically illustrating a cross connect settingin a communication node #5 illustrated in FIG. 1;

FIG. 8 is a diagram schematically illustrating a cross connect settingin a communication node #7 illustrated in FIG. 1;

FIG. 9 is a diagram schematically illustrating a cross connect settingin a communication node #4 illustrated in FIG. 1;

FIG. 10 is a diagram schematically illustrating a cross connect settingin a communication node #9 illustrated in FIG. 1;

FIG. 11 is a network configuration diagram for describing an example ofpath switching control when a failure occurs in one of working paths inthe network configuration illustrated in FIG. 1 (when a single failureoccurs);

FIG. 12 is a diagram schematically illustrating a cross connect settingin a communication node #1 illustrated in FIG. 11;

FIG. 13 is a diagram schematically illustrating a cross connect settingin a communication node #8 illustrated in FIG. 11;

FIG. 14 is a diagram schematically illustrating a cross connect settingin a communication node #5 illustrated in FIG. 11;

FIG. 15 is a diagram schematically illustrating a cross connect settingin a communication node #7 illustrated in FIG. 11;

FIG. 16 is a diagram schematically illustrating a cross connect settingin a communication node #4 illustrated in FIG. 11;

FIG. 17 is a diagram schematically illustrating a cross connect settingin a communication node #9 illustrated in FIG. 11;

FIG. 18 is a flowchart for describing an exemplary operation of thecommunication node #1 illustrated in FIG. 11;

FIG. 19 is a flowchart for describing an exemplary operation of thecommunication node #5 illustrated in FIG. 11;

FIG. 20 is a flowchart for describing an exemplary operation of thecommunication node #4 illustrated in FIG. 11;

FIG. 21 is a flowchart for describing an exemplary operation of thecommunication node #9 illustrated in FIG. 11;

FIG. 22 is a network configuration diagram for describing an example ofpath switching control when a failure occurs in each of a plurality ofworking paths in the exemplary network configuration illustrated in FIG.1 (when multiple failures occur);

FIG. 23 is a diagram schematically illustrating a cross connect settingin a communication node #1 illustrated in FIG. 22;

FIG. 24 is a diagram schematically illustrating a cross connect settingin a communication node #8 illustrated in FIG. 22;

FIG. 25 is a diagram schematically illustrating a cross connect settingin a communication node #5 illustrated in FIG. 22;

FIG. 26 is a diagram schematically illustrating a cross connect settingin a communication node #7 illustrated in FIG. 22;

FIG. 27 is a diagram schematically illustrating a cross connect settingin a communication node #4 illustrated in FIG. 22;

FIG. 28 is a diagram schematically illustrating a cross connect settingin a communication node #9 illustrated in FIG. 22;

FIG. 29 is a flowchart for describing an exemplary operation of thecommunication node #5 illustrated in FIG. 22;

FIG. 30 is a network configuration diagram for describing an example ofpath switching control when a failure of one working path is restored(or recovered) in the network configuration illustrated in FIG. 22;

FIG. 31 is a diagram schematically illustrating a cross connect settingin a communication node #1 illustrated in FIG. 30;

FIG. 32 is a diagram schematically illustrating a cross connect settingin a communication node #8 illustrated in FIG. 30;

FIG. 33 is a diagram schematically illustrating a cross connect settingin a communication node #5 illustrated in FIG. 30;

FIG. 34 is a diagram schematically illustrating a cross connect settingin a communication node #7 illustrated in FIG. 30;

FIG. 35 is a diagram schematically illustrating a cross connect settingin a communication node #4 illustrated in FIG. 30;

FIG. 36 is a diagram schematically illustrating a cross connect settingin a communication node #9 illustrated in FIG. 30;

FIG. 37 is a diagram illustrating an exemplary network configuration ina shared restoration scheme;

FIG. 38 is a diagram illustrating an exemplary network configurationbefore a failure occurs; and

FIG. 39 is a diagram illustrating an exemplary network configurationwhen a single failure occurs in the network configuration illustrated inFIG. 38.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment will be described with reference to theappended drawings. Here, the following descriptions are merely anexample and not intended to exclude applications of variousmodifications or techniques which are not described below. In thedrawings used in the following embodiment, parts denoted by the samereference numerals represent the same or similar parts unless otherwiseset forth herein.

FIG. 1 is a diagram illustrating an exemplary configuration of acommunication network according to an embodiment. The communicationnetwork illustrated in FIG. 1 includes, for example, a plurality ofcommunication nodes #1 to #11 each of which serves as an example of atransmission apparatus. The communication nodes #1 to #11 are connected,for example, in a mesh form and form a mesh network.

The communication node #1 corresponds to a source node of the workingpath A and the protection path A for the working path A. The workingpath A is an example of a first working path, and is set to start fromthe source node #1 and arrive at the communication node #4 through thecommunication node #2 and the communication node #3. In other words, ineach of the communication nodes #1, #2, #3, and #4, a physicalconnection setting (for example, a cross connect setting) on the workingpath A is available.

The communication node #8 corresponds to a source node of the workingpath B and the protection path B for the working path B. The workingpath B is an example of a second working path, and is set to start fromthe source node #8 and arrive at the communication node #9 through thecommunication node #10 and the communication node #11. In other words,in each of the communication nodes #8, #10, #11, and #9, a physicalconnection setting (for example, a cross connect setting) on the workingpath B is available.

The protection path A is, for example, set to start from the source node#1 of the working path A and arrive at the communication node #4corresponding to the end node of the working path A through thecommunication nodes #5, #6, and #7. In other words, a physicalconnection is secured in the protection path A, and it is available totransmit a signal through the protection path A.

Here, during the normal operation, in the source node #1, a physicalconnection setting on the protection path A has not been made (see adotted line) but a band reservation for the protection path A has beenmade. When a failure occurs in the working path A, the source node #1releases the physical connection setting on the working path A andperforms the physical connection setting using the reserved band for theprotection path A. Thereby, the signal having been transmitted to theworking path A can be transmitted to the protection path A (thecommunication node #5).

Similarly, the protection path B is set to start from the source node #8of the working path B and arrive at the communication node #9corresponding to the end node of the working path B through thecommunication nodes #5, #6, and #7. In other words, a physicalconnection is secured in the protection path B, and it is available totransmit a signal through the protection path B.

However, during the normal operation, in the source node #8, a physicalconnection setting on the protection path B has not been made (see adotted line), but a band reservation for the protection path B has beenmade. When a failure occurs in the working path B, the source node #8releases the physical connection setting on the working path B andperforms a physical connection setting using the reserved band for theprotection path B. Thereby, the signal transmitted to the working path Bcan be transmitted to the protection path B (the communication node #5).

The protection path A and the protection path B are partially common,for example, between the communication nodes #5 and #7. In other words,the communication node #5 corresponds to the source node of a commonpath C serving as a protection path shared by the working paths A and B.Further, the communication node #7 corresponds to the end node of thecommon path C. A physical connection has been set to the common path C.

In other words, between the communication node #5 serving as the sourcenode of the common path C and the communication node #6 serving as anintermediate node of the common path C, the physical connection settinghas been made so that a signal can be communicated through the commonpath C. Similarly, between the intermediate node #6 of the common path Cand the communication node #7 serving as the end node of the common pathC, the physical connection setting has been made so that a signal can becommunicated through the common path C.

The source node #5 of the common path C transmits one of signals each ofwhich received through a plurality of protection paths A and B to thecommon path C. In other words, the source node #5 of the common path Cconnects one of the protection paths A and B to the common path C. Thus,in the common path C, one of the signal of the protection paths A and Bis transmitted toward the communication node #7.

In the end node #7 of the common path C, a physical connection settinghas been made on each of the end node #4 of the working path A and theend node #9 of the working path B. In other words, the end node #7 ofthe common path C is in a state available to transmit the signal to eachof the end nodes #4 and #9. At this time, the end node #7 of the commonpath C is operable to copy the signal received through the common path Cand to transmit the signal to each of the end nodes #4 and #9.

The end node #4 of the working path A is operable to receive the signaltransmitted from the working path A (the communication node #3) and thesignal transmitted from the protection path A through the common path C(the end node #7) and to output one of the received signals. Similarly,the end node #9 of the working path B is operable to receive the signaltransmitted from the working path B (the communication node #11) and thesignal transmitted from the protection path B through the common path C(the communication node #7) and to output one of the received signals.

FIG. 1 illustrates the two communication nodes #2 and #3 each of whichserves as the intermediate node through which the working path A passesHowever, three or more intermediate nodes may be provided or nointermediate node may be provided for the working path A. Similarly,three or more intermediate nodes may be provided or no intermediate nodeprovided for the working path B. Further, FIG. 1 illustrates the singlecommunication node #6 serving as the intermediate node through which thecommon path C passes. However, two or more intermediate nodes may beprovided or no intermediate node provided for the common path C.

In the network configuration described above, during the normaloperation, the communication node #1 transmits a received user signal(user traffic) to the working path A, and the communication node #8transmits a received user traffic to the working path B. The usertraffic is an example of a client signal which may correspond to, forexample, an ODUk (k=0, 1, 2, 3, 4, and so on) mapped to an OTN frame.

Meanwhile, the communication node #1 transmits an alarm signal to theprotection path A, and the communication node #8 transmits the alarmsignal to the protection path B. The alarm signal is an example of adummy signal which may correspond to, for example, an open connectionindication—ODUk (OCI-ODUk) alarm representing that the user traffic isin a non-connection status.

Thus, during the normal operation, the communication node #5 receivesthe alarm signal (for example, the OCI-ODUk alarm) from each of thesource nodes #1 and #8 of the respective working paths A and B. Thecommunication node #5 selects one of the received alarm signalsaccording to a predetermined priority and transmits the selected alarmsignal to the common path C. In the example of FIG. 1, the communicationnode #5 selects the OCI-ODUk alarm received through the protection pathA and transmits the OCI-ODUk alarm to the common path C.

The OCI-ODUk alarm is transmitted through the common path C in the orderof the communication nodes #6 and #7. The end node #7 of the common pathC copies the OCI-ODUk alarm received through the common path C andtransmits the OCI-ODUk alarm to each of the end nodes #4 and #9 of theworking paths A and B.

Thereby, the end node #4 of the working path A receives the user trafficfrom the working path A (the communication node #3) and receives thealarm signal from the protection path A (the end node #7 of the commonpath C). The end node #4 selects the user traffic received from theworking path A, which is not an alarm signal, and outputs the selecteduser traffic.

Similarly, the end node #9 of the working path B receives the usertraffic from the working path B (the communication node #11) andreceives the alarm signal from the protection path B (the end node #7 ofthe common path C). The end node #9 selects the user traffic receivedfrom the working path B, which is not an alarm signal, and outputs theselected user traffic.

In the above example, the source node #1 (or #8) of the working path A(or B) transmits the alarm signal that is an example of the dummy signalto the protection path A (or B). However, the source node #1 (or #8) maytransmit a signal which is not an alarm signal but, for example, thesame signal as the signal transmitted to the working path A (or B).

(Exemplary Configuration of Communication Node)

FIG. 2 is a diagram illustrating an exemplary hardware configuration ofthe communication node #j (j=any one of 1 to 11) illustrated in FIG. 1,and FIG. 3 is a functional block diagram illustrating the communicationnode #j illustrated in FIG. 1.

The communication node #j illustrated in FIG. 2 includes, for example, aplurality of OTN interface cards (OTN interface units) 20 each of whichan optical fiber (optical transmission line) 40 is connected to and anOTN switch card (OTN switch unit) 30 in a shelf 10. Each of the OTNinterface cards 20 is connected with the OTN switch card 30 through aninterconnection 50. An optical signal inputted to a first OTN interfacecard 20 through any one of optical fibers 40 is switched its outputdestination corresponding to another second OTN interface card 20 at theOTN switch card 30. The switched signal is outputted from the second OTNinterface card 20 to the corresponding optical fiber 40.

For this, the OTN switch card 30 includes, for example, a matrix switch(an optical switch functioning unit) 31, and the OTN interface card 20includes, for example, an optical transceiver 21 and an OTN framer 22.The OTN framer 22 can be implemented, for example, using an LSI or anFPGA.

The OTN framer 22 performs multi-mapping (or multiplexing) andmulti-demapping (or demultiplexing) on an ODUk frame compatible withITU-T recommendation G.709. For example, an ODU frame to which a usertraffic of Ethernet (a registered trademark) is mapped may be referredto as a lower order ODU (LO-ODU), and an ODU frame to which a pluralityof ODU frames is mapped may be referred to as a higher order ODU(HO-ODU). For example, ODU1 may be multiplexed into a HO-ODUk frame (forexample, ODU2, ODU3, ODU4, and so on) which has a signal speed higherthan the ODU 1.

FIG. 4 illustrates an exemplary format of an ODUk frame compatible withITU-T recommendation G.709. The ODUk frame includes an overhead fieldwith the size of 16 bytes of 1st to 16-th columns×4 rows and a payloadfield referred to as an optical channel payload unit k (OPUk) with thesize of 3808 bytes of 17-th to 3824-th columns×4 rows.

The overhead field includes a frame alignment (FA) overhead, an OTUkoverhead, an ODUk overhead, and an optical channel payload unit k (OPUk)overhead, and is used for connection and quality management. The ODUkmay be mapped into any one of time slots (tributary slots (TS)) obtainedby dividing a payload field (an OPUk payload area) of a HO-ODUk frame,for example, in units of bytes. Thereby, the mapping of the ODUk intothe HO-ODUk can be achieved.

Abbreviation terms indicated in FIG. 4 mean the followings:

PM: Path Monitoring

TCM: Tandem Connection Monitoring

RES: Reserved for future international standardization

ACT: Activation/deactivation control channel

FTFL: Fault Type & Fault Location reporting channel

EXP: Experiment

GCC: General Communication Channel

APS: Automatic Protection Switching coordination channel

PCC: Protection Communication Control channel

In the present embodiment, as will be described later, the source nodes#1 and #8 insert link identification information (link ID) of the usertraffic into the RES field. Thus, each of the end nodes #4 and #9 isavailable to determine whether or not the user traffic is allowed totransfer based on the link ID inserted in the RES field.

Focusing on functions of the communication node #j, the communicationnode #j includes, for example, an OTN interface unit 20, a managementcontrol functioning unit 60, and an OTN switch unit 30, as illustratedin FIG. 3.

The OTN interface unit 20 includes an optical transceiver 21, the OTNframer 22, an alarm inserting unit 23, a link ID inserting unit 24, atransceiver 25, an alarm detector 26, and a link ID detector 27.

The management control functioning unit 60 controls the physicalconnection setting (the cross connect setting or the like) on the OTNswitch unit 30. For example, in the communication node #5 serving as thesource node of the common path C and the communication node #7 servingas the end node of the common path C, the management control functioningunit 60 functions as an example of a connection setting unit operable tosecure the physical connection setting on the common path C.

For example, the management control functioning unit 60 includes analarm status manager 61, a link ID manager 62, and a link prioritymanager 63.

The OTN switch unit 30 performs the cross connect setting or the likeaccording to the connection setting given by the management controlfunctioning unit 60. In the source node #5 of the common path C, the OTNswitch unit 30 functions as an example of a signal transfer unitoperable to transfer one of the signals (the first and second signals)received from the source node #1 and #8 (the first and second nodes) ofthe working paths A and B to the common path C (the communication node#5). Further, in the end node #7 of the common path C, the OTN switchunit 30 functions as an example of a transmitter operable to transmitthe signal received from the communication node #6 at the upstream sideto each of the end nodes #4 and #9 of the working paths A and B throughthe common path C.

The OTN switch unit 30 includes, for example, an optical switchfunctioning unit 31. The optical switch functioning unit 31 includes asplitter and a selector as will be described later.

In the OTN interface unit 20, the optical transceiver 21 transceiver theOTN frame including the user traffic (client signal).

The OTN framer 22 is operable to perform the multi-mapping (ormultiplexing) and multi-demapping (or demultiplexing) on the ODUk frameas described previously.

The alarm inserting unit 23 is operable to insert an alarm signal suchas an OCI-ODUk alarm, a backward defect indication-ODUk (BDI-ODUk) alarmwhich will be described later, or an AIS-ODUk alarm into an OTN frame.

The OCI-ODUk alarm may be indicated, for example, by a signal in whichthe entire of the ODUk frame other than an FA overhead, an OTUkoverhead, and an FTFL field which are illustrated in FIG. 4 is set to apredetermined repetition pattern (for example, “0110 0110”).

The BDI-ODUk alarm is inserted into a section monitoring (SM) fielddefined in the FA overhead illustrated in FIG. 4 and indicates a signalfailure (backward defect) with “1”, for example.

The AIS-ODUk alarm is indicated by a signal in which the entire of theODUk frame other than the FA overhead, the OTUk overhead, and the FTFLfield illustrated in FIG. 4 is set to all “1”.

The link ID inserting unit 24 functions as an example of a linkidentification information allocator operable to insert (or allocate) alink ID related to the working path A or B into the user traffic (clientsignal). The link ID inserting unit 24 is operable to insert the link IDinto the RES field as described previously. For example, the link ID maybe managed by the link ID manager 62 of the management controlfunctioning unit 60.

The transceiver 25 is operable to transceive a signal between the OTNinterface unit 20 and the OTN switch unit 30.

The alarm detector 26 is operable to detect the OCI-ODUk alarm, theBDI-ODUk alarm, and the AIS-ODUk. An alarm detection result (alarmstatus) indicating whether the alarm is detected or not may be notifiedto the alarm status manager 61 of the management control functioningunit 60. In the source nodes #1 and #8 of the working paths A and B, thealarm detector 26 functions as an example of a failure detector operableto detect the BDI-ODUk alarm to thereby detect a failure of the workingpath.

The link ID detector 27 is operable to detect the link ID inserted intothe RES field of the received user traffic. The detected link ID may benotified to the link ID manager 62 of the management control functioningunit 60.

Focusing on the source nodes #1 and #8 of the working paths A and B, theoptical transceiver 21, the OTN framer 22, the transceiver 25, and theOTN switch unit 30 function as an example of first and secondtransmitters. The first transmitter is operable to transmit the clientsignal to the working path A or B. The second transmitter is operable totransmit the OCI-ODUk alarm that is an example of the dummy signal tothe common path C (the communication node #5) serving as the protectionpath shared by the working path A (or B) and another working path B (orA). Further, the second transmitter is operable to stop transmitting theOCI-ODUk alarm and to transmit the received client signal to the sourcenode #5 of the common path C, in response to a detection of the BDI-ODUkalarm in the alarm detector 26.

Meanwhile, focusing on the end nodes #4 and #9 of the working paths Aand B, the OTN interface unit 20 has a function as a first receiveroperable to receive a first signal transmitted through the working pathA or B. Further, the OTN interface unit 20 has a function as a secondreceiver operable to receive a second signal transmitted through thecommon path C serving as the protection path shared by the working pathA (or B) and another working path B (or A). In this case, the OTN switchunit 30 functions as an example of a signal transfer unit operable totransfer one of the first and second signals received by the first andsecond receivers. Further, the second receiver operable to receive, fromthe common path C, the same signal as the signal transmitted to anothernode node#9 (or #4) that receives the signal transmitted through anotherworking path B (or A).

Further, focusing on the source node #5 of the common path C, the OTNinterface unit 20 has a function as a first receiver operable to receivea first signal from the first node #1 that transmits the signal to thefirst working path A. Further, the OTN interface unit 20 has a functionas a second receiver operable to receive a second signal from the secondnode #8 that transmits the signal to the second working path B. Further,the first receiver is operable to receive the client signal transmittedthrough the first working path A from the first node #1 in response to afailure occurring in the first working path A. Upon receiving the clientsignal and the second signal, the signal transfer unit is operable totransfer the client signal to the common path C.

Further, focusing on the end node #7 of the common path C, the OTNinterface unit 20 has a function as a receiver operable to receive thesignal transmitted through the common path C serving as the protectionpath shared by the first working path A and the second working path B.The OTN switch unit 30 of the end node #7 functions as an example of atransmitter operable to transmit the signal received by the receiver toboth of the communication node #4 and communication node #9. In thiscase, the communication node #4 corresponds to the first node thatreceives the signal transmitted through the first working path A, andthe communication node #9 corresponds to the second node that receivesthe signal transmitted through the second working path B.

Next, in the management control functioning unit 60, the alarm statusmanager 61 is operable to manage the alarm detection result obtained bythe alarm detector 26. The alarm status manager 61 is also operable totransmit a path switching request to the optical switch functioning unit31 of the OTN switch unit 30 according to the alarm detection result.

For example, when the AIS-ODUk alarm or the BDI-ODUk alarm is detectedin the working path A (or B), a request to switch the working path A (orB) to the protection path A (or B) is transmitted to the optical switchfunctioning unit 31. Further, for example, when the AIS-ODUk alarm orthe BDI-ODUk alarm is canceled in the working path A (or B), a requestto switch (switch back) the protection path A (or B) to the working pathA (or B) is transmitted to the optical switch functioning unit 31.

When the OCI-ODUk alarm is detected, the alarm status manager 61transfers a priority determination request to the link priority manager63.

The link priority manager 63 is operable to determine (select) a pathsignal to be transferred to the downstream side according to apredetermined link priority and to transfer the path switching requestaccording to the selection result to the optical switch functioning unit31, in response to a reception of the priority determination requesttransmitted from the alarm status manager 61.

For example, since the communication node #5 illustrated in FIG. 1receives the OCI-ODUk alarm through both of the protection paths A andB, the link priority manager 63 selects a protection path through whichthe received alarm is to be transferred to the downstream communicationnode #6 according to the link priority. In the example of FIG. 1, sincethe protection path A is higher in the link priority than the protectionpath B, the protection path A is selected. Further, in the end nodes #4and #9 illustrated in FIG. 1, the link priority manager 63 selects theworking paths A and B from which the OCI-ODUk alarm is not received.

The link ID manager 62 is operable to manage an link ID transferable tothe downstream, to compare the link ID with the link ID notified fromthe link ID detector 27, and to determine whether the user trafficincluding the corresponding link ID is allowed to transfer to thedownstream (whether to transfer or not). The link ID manager 62 is alsooperable to give a path switching request to the optical switchfunctioning unit 31 according to the determination result.

For example, as will be described later, there are cases in which afailure occurs in either or both of the working paths A and Billustrated in FIG. 1 and the user traffic including the same link ID istransferred from the communication node #7 to each of the end nodes #4and #9. In this case, the end nodes #4 and #9 select a path signal to betransferred to the downstream based on the link ID.

Next, in the OTN switch unit 30, the optical switch functioning unit 31is operable to perform the cross connect setting for input and outputsignals according to the path switching request given from themanagement control functioning unit 60. The cross connect setting isavailable for TS units of the ODUk frame.

As illustrated in FIGS. 5 to 10, the optical switch functioning unit 31includes a splitter 311 and a selector 312. The splitter 311 splits thereceived signal into signals inputted to a plurality of input ports ofthe selector 312, and the selector 312 selects a signal to be output.FIG. 5 schematically illustrates the cross connect setting in thecommunication node #1 illustrated in FIG. 1, FIG. 6 schematicallyillustrates the cross connect setting in the communication node #8illustrated in FIG. 1, and FIG. 7 schematically illustrates the crossconnect setting in the communication node #5 illustrated in FIG. 1.Further, FIG. 8 schematically illustrates the cross connect setting inthe communication node #7 illustrated in FIG. 1, FIG. 9 schematicallyillustrates the cross connect setting in the communication node #4illustrated in FIG. 1, and FIG. 10 schematically illustrates the crossconnect setting in the communication node #9 illustrated in FIG. 1.

As illustrated in FIG. 5, the source node #1 of the working path Atransfers an ODU2 frame (a user traffic A) mapped in an OTU2 framereceived from the upstream side to the working path A. FIG. 5illustrates an example in which the user traffic A is mapped into, forexample, TSs 3-10 of LO-ODU2 the LO-ODU2 is included in an OTU4 frame,and the OTU4 is transferred. Meanwhile, the source node #1 transmits anOCI-ODU2 alarm to the protection path B as described previously.

Similarly, as illustrated in FIG. 6, the source node #8 of the workingpath B transfers an ODU2 frame (a user traffic B) mapped in the OTU2frame received from the upstream side to the working path B. Meanwhile,the source node #8 transmits the OCI-ODU2 alarm to the protection path Bas described previously.

Further, as illustrated in FIG. 7, the source node #5 of the common pathC selects one (for example, the OCI-ODU2 alarm received from theprotection path A) of the OCI-ODU2 alarms each of which received throughboth of the protection paths A and B to transfer the selected OCI-ODU2alarm to the downstream communication node #6.

Further, as illustrated in FIG. 8, the end node #7 of the common path Ccopies the OCI-ODU2 alarm received from the upstream communication node#6 and transfers the OCI-ODU2 alarms to the respective end nodes #4 and#9 of the working paths A and B.

Further, as illustrated in FIG. 9, the end node #4 of the working path Aselects the user traffic A among the user traffic A received from theupstream communication node #3 and the OCI-ODU2 alarm received from thecommunication node #7 to thereby transfer the selected user traffic A tothe downstream.

Further, as illustrated in FIG. 10, the end node #9 of the working pathB selects the user traffic B among the user traffic B received from theupstream communication node #11 and the OCI-ODU2 alarm received from thecommunication node #7 to thereby transfer the user traffic B to thedownstream.

(When Single Failure Occurs)

Next, the description will proceed with an example of path switchingcontrol when a failure occurs in one of the working paths A and B, forexample, the working path A (between the communication nodes #2 and #3)in the network configuration and the communication node configurationdescribed above, as illustrated in FIG. 11. FIGS. 12 to 17 schematicallyillustrate examples of the cross connect setting in the communicationnodes #1, #8, #5, #7, #4, and #9, respectively, when the failure occurs.

When the signal transmission failure occurs in the working path Abetween the communication nodes #2 and #3, the downstream communicationnode #3 detects the occurrence of the failure because the ODUk frame isnot received. The communication node #3 detecting the occurrence of thefailure generates the AIS-ODUk alarm and transmits the AIS-ODUk alarm tothe downstream communication node #4. Meanwhile, the communication node#3 generates the BDI-ODUk alarm and transmits the BDI-ODUk alarm to theupstream communication node #2. Although not illustrated in FIG. 11, theBDI-ODUk alarm is transmitted to the upstream through an opticaltransmission line which is paired with a failure occurred opticaltransmission line and transmits an optical signal in a directionopposite to that of the working path A. A two-way optical transmissionline is provided similarly between the other communication nodes #j (thesame applies hereinafter).

In response to a reception of the BDI-ODUk alarm from the downstreamcommunication node #3, the communication node #2 further transfers thereceived BDI-ODUk alarm to the upstream communication node #1.

In the source node #1 of the working path A and the protection path A,the optical transceiver 21 of the OTN interface unit 20 connected to theworking path A receives the BDI-ODUk alarm (process P₁₁ of FIG. 18), andthe alarm detector 26 detects the BDI-ODUk alarm.

The alarm detector 26 notifies the alarm status manager 61 of thedetection of the BDI-ODUk alarm (process P₁₂ of FIG. 18). The alarmstatus manager 61 requests the optical switch functioning unit 31 toperform switching to the protection path A (process P₁₃ of FIG. 18). Theoptical switch functioning unit 31 performs switching to the protectionpath A in response to the switching request (process P₁₄ of FIG. 18).

In other words, the communication node #1 releases the physicalconnection setting (the cross connect setting or the like) on theworking path A and performs the physical connection setting on theprotection path A. Thereby, the user traffic A is transmitted to theprotection path A (see FIG. 12). Here, the source node #1 inserts thelink ID (for example, the link A) of the user traffic A into the RESfield of the user traffic A by the link ID inserting unit 24 of the OTNinterface unit 20 connected to the protection path A (process P₁₅ ofFIG. 18).

The ODUk frame of the user traffic A into which the link ID is insertedis transmitted from the optical transceiver 21 of the OTN interface unit20 connected to the protection path A to the protection path A (processP₁₆ of FIG. 18). Therefore, an ID of the link A is included in the ODUkframe of the user traffic A transmitted from the source node #1 to theprotection path A.

Meanwhile, the source node #8 of the working path B and the protectionpath B transmits the OCI-ODUk alarm to the protection path B whilemaintaining the physical connection setting on the working path B (seeFIG. 13). Here, an ID of a link B is inserted by the link ID insertingunit 24 into the ODUk frame of the traffic transmitted from thecommunication node #8 to the working path B.

The source node #5 of the common path C becomes unavailable to receivethe OCI-ODUk alarm through the protection path A (cancellation ofalarm). Instead, the communication node #5 receives the user traffic Athrough the optical transceiver 21 of the OTN interface unit 20connected to the protection path A (process P₂₁ of FIG. 19). Meanwhile,the communication node #5 receives the OCI-ODUk alarm through theoptical transceiver 21 of the OTN interface unit 20 connected to theprotection path B.

Thus, the alarm detector 26 detects the cancellation of the OCI-ODUkalarm on the protection path A, and the alarm detector 26 notifies thealarm status manager 61 of the cancellation of the OCI-ODUk alarm on theprotection path A (process P₂₂ of FIG. 19). The alarm status manager 61that has been notified of the cancellation of the OCI-ODUk alarmrequests the optical switch functioning unit 31 to perform switching tothe protection path A (process P₂₃ of FIG. 19).

The optical switch functioning unit 31 performs switching to theprotection path A in response to the switching request (process P₂₄ ofFIG. 19). As described above, in response to the cancellation of theOCI-ODUk alarm of the protection path A, the communication node #5selects by using the selector 312 the user traffic A received throughthe protection path A to be transferred (see FIG. 14). Thereby, the usertraffic A is transmitted from the OTN interface unit 20 connected to thecommon path C to the common path C (process P₂₅ of FIG. 19). As aresult, the user traffic A is transferred through the common path C inthe order of the communication nodes #6 and #7.

The end node #7 of the common path C copies the user traffic A receivedfrom the communication node #6 using the splitter 311 and transmits theuser traffic A to both of the end nodes #4 and #9 of the working paths Aand B (see FIG. 15).

The end node #4 of the working path A receives the AIS-ODUk alarm fromthe communication node #3 through the working path A. Meanwhile, the endnode #4 receives the user traffic A including the link ID of the link Afrom the protection path A (the common path C) through the opticaltransceiver 21 of the OTN interface unit 20 connected with thecommunication node #7 (process P₃₁ of FIG. 20).

The received user traffic A is transferred to the link ID detector 27,and the link ID detector 27 detects the link ID (the link A) of the usertraffic A and notifies the link ID manager 62 of the detected link ID(process P₃₂ of FIG. 20).

The link ID manager 62 determines that the received user traffic A isallowed to transfer because the notified link ID is the link A. Thus,the link ID manager 62 transmits the switching request to the opticalswitch functioning unit 31 to transfer the user traffic A received fromthe communication node #7 (process P33 of FIG. 20). Thereby, the opticalswitch functioning unit 31 is switched to select the user traffic A tobe transferred (process P34 of FIG. 20), and the user traffic A receivedfrom the communication node #7 is transferred to the downstream (processP35 of FIG. 20 and see FIG. 16).

Meanwhile, the end node #9 of the working path B receives the usertraffic B including the link ID of the link B from the communicationnode #11 through the working path B. The end node #9 also receives theuser traffic A including the link ID of the link A from the protectionpath B (the common path C) through the optical transceiver 21 of the OTNinterface unit 20 connected with the communication node #7 (process P41of FIG. 21).

The received user traffic A is transferred to the link ID detector 27,and the link ID detector 27 detects the link ID (the link A) of the usertraffic A and notifies the link ID manager 62 of the detected link ID(process P42 of FIG. 21).

The link ID manager 62 determines whether the received user traffic isallowed to transfer based on the notified link ID. In this case, thelink ID manager 62 determines that the user traffic B including the linkID of the link B is allowed to transfer. Thus, the link ID manager 62transmits the switching request to the optical switch functioning unit31 to transfer the user traffic B. Thereby, the communication node #9selects by using the selector 312 the user traffic B received throughthe working path B to be transferred (process P43 of FIG. 21 and seeFIG. 17).

As described above, even when the signal transmission failure occurs inthe working path A, since the user traffic A and the user traffic B arenormally transmitted through the protection path A and the working pathB, respectively, the failure of the working path A is restored by theprotection path A.

Further, when the signal transmission failure occurred in the workingpath B, in a manner similar to the above-described example, the usertraffic A and the user traffic B can be normally transmitted through theworking path A and the protection path B, respectively.

In other words, in response to a reception of the BDI-ODUk alarm fromthe downstream communication node #10, the source node #8 of the workingpath B cancels the transmission of the OCI-ODUk alarm to the protectionpath B (the communication node #5) and switches the transmissiondestination of the user traffic B to the protection path B. The usertraffic B includes the link ID of the link B.

Since the communication node #5 receives the OCI-ODUk alarm from theprotection path A and receives the user traffic B from the protectionpath B, the communication node #5 selects the user traffic B receivedfrom the protection path B to be transferred. Thereby, the user trafficB is transferred to the communication node #7 through the communicationnode #6.

The communication node #7 copies the user traffic B received from theupstream communication node #6 and transfers the user traffic B to therespective downstream communication nodes #4 and #9.

The communication node #4 serving as the end node of the working path Areceives the user traffic A including the link ID of the link A from theworking path A (from the communication node #3) and receives the usertraffic including the link ID of the link B from the protection path A(from the end node #7 of the common path C). The communication node #4determines that the user traffic A including the link ID of the link Ais a user traffic allowed to transfer to thereby select the user trafficA to be transferred.

The communication node #9 serving as the end node of the working path Breceives the AIS-ODUk alarm from the working path B and receives theuser traffic B including the link ID of the link B from the protectionpath B (from the end node #7 of the common path C). The communicationnode #9 selects the user traffic including the link ID of the link B tobe transferred.

Thereby, even when the signal transmission failure occurs in the workingpath B, since the user traffic B and the user traffic A are normallytransmitted through the protection path B and the working path A,respectively, the failure of the working path B is restored by theprotection path B.

(When Multiple Failures Occur)

Next, an example of path switching control when a signal transmissionfailure occurs in both of the working paths A and B as illustrated inFIG. 22 will be described. FIGS. 23 to 28 schematically illustrate crossconnect setting examples in the communication node #1, #8, #5, #7, #4,and #9, respectively, when the multiple failures occur.

As described previously, when a signal transmission failure occurs inthe working path A between the communication nodes #2 and #3, theoccurrence of the failure is detected since the ODUk frame is notreceived in the downstream communication node #3. The communication node#3 detecting the failure generates the AIS-ODUk alarm and transmits theAIS-ODUk alarm to the downstream communication node #4. Meanwhile, thecommunication node #3 generates the BDI-ODUk alarm and transmits theBDI-ODUk alarm to the upstream communication node #2.

Similarly, when a signal transmission failure occurs in the working pathB between the communication nodes #10 and #11, the occurrence of thefailure is detected since the ODUk frame is not received in thedownstream communication node #11. The communication node #11 detectingthe failure generates the AIS-ODUk alarm and transmits the AIS-ODUkalarm to the downstream communication node #9. Meanwhile, thecommunication node #11 generates the BDI-ODUk alarm and transmits theBDI-ODUk alarm to the upstream communication node #10.

The BDI-ODUk alarm on the working path A is transferred to thecommunication node #1 through the communication node #2, and theBDI-ODUk alarm on the working path B is transferred to the communicationnode #8 through the communication node #10.

Upon detecting a reception of the BDI-ODUk alarm through the workingpath A, the communication node #1 serving as the source node of theworking path A and the protection path A releases the physicalconnection setting (the cross connect setting or the like) on theworking path A. Then, the source node #1 performs the physicalconnection setting on the protection path A and transmits the usertraffic A to the protection path A (see FIG. 23). Here, the ODUk frameof the user traffic A transmitted from the source node #1 to theprotection path A includes, for example, an ID of the link A.

Meanwhile, upon detecting a reception of the BDI-ODUk alarm through theworking path B, the communication node #8 serving as the source node ofthe working path B and the protection path B releases the physicalconnection setting (the cross connect setting or the like) on theworking path B. Then, the source node #8 performs the physicalconnection setting on the protection path B and transmits the usertraffic B to the protection path B (see FIG. 24). Here, the ODUk frameof the user traffic B transmitted from the source node #8 to theprotection path B includes, for example, an ID of the link B.

In the communication node #5 serving as the source node of the commonpath C shared by the protection path A and the protection path B, theOCI-ODUk alarm is not received from any of the protection path A and theprotection path B (the alarm signal has been cancelled). Instead, thecommunication node #5 receives the user traffics A and B through theoptical transceiver 21 of the OTN interface unit 20 connected to theprotection paths A and B (processes P51 and P52 of FIG. 29).

Thus, the alarm detector 26 detects the cancellation of the OCI-ODUkalarm and notifies the alarm status manager 61 of the cancellation(processes P53 and P54 of FIG. 29). In response to a reception of thecancellation notification of the OCI-ODUk alarm, the alarm statusmanager 61 requests the link priority manager 63 to perform a linkpriority determination (process P55 of FIG. 29).

The link priority manager 63 selects a path (user traffic) to betransferred according to a priority of the protection paths A and B. Forexample, when the protection path A is higher in priority than theprotection path B, the link priority manager 63 transmits the switchingrequest to the optical switch functioning unit 31 to select the signalfrom the protection path A to be transferred (the user traffic A)(process P56 of FIG. 29).

The optical switch functioning unit 31 performs switching so that theuser traffic A received through the protection path A is transferredaccording to the switching request (process P57 of FIG. 29 and see FIG.25). Thereby, the user traffic A is transferred to the communicationnode #6 through the OTN interface unit 20 connected to the common path C(process P58 of FIG. 29).

Meanwhile, the communication node #5 generates the BDI-ODUk alarm andtransmits the BDI-ODUk alarm to the communication node #8 serving as thesource node of the working path B (see FIG. 22). Thereby, thecommunication node #8 serving as the source node of the working path Bcan recognize that a connection (a restoration) to the protection path Bfor the working path A has been failed.

The end node #7 of the common path C copies the user traffic A receivedfrom the communication node #6 using the splitter 311 and transmits theuser traffic A to the respective communication nodes #4 and #9 servingas the end node of the working paths A and B (see FIG. 26).

The end node #4 of the working path A receives the AIS-ODUk alarm fromthe working path A and receives the user traffic A including the link IDof the link A in the ODUk frame from the protection path A (from the endnode #7 of the common path C). Thus, the communication node #4 selectsthe user traffic A to be transferred, which is received from thecommunication node #7 by using the selector 312 (see FIG. 27).

The communication node #9 serving as the end node of the working path Breceives the AIS-ODUk alarm from the working path B and receives theuser traffic A including the link ID of the link A in ODUk frame fromthe protection path B (from the end node #7 of the common path C). Sincethe link ID does not indicate the user traffic allowed to transfer, thecommunication node #9 selects the AIS-ODUk alarm to be transferred,which is received from the working path B (see FIG. 28).

As described above, even when the signal transmission failure occurs inboth of the working paths A and B, at least one of user traffics can berestored according to the priority of the protection paths A and B.

(When Failure is Restored)

Next, an example of path switching control when the signal transmissionfailure of one working path (for example, the working path A) isrestored as illustrated in FIG. 30 from a state in which the signaltransmission failures occurred in both of the working paths A and B asdescribed above will be described. FIGS. 31 to 36 schematicallyillustrate cross connect setting examples in the communication nodes #1,#8, #5, #7, #4, and #9, respectively, when a failure is restored.

When the signal transmission failure between the communication nodes #2and #3 is restored, the communication node #3 cancels transmission ofthe BDI-ODUk alarm to the upstream and transmission of the AIS-ODUkalarm to the downstream.

Thereby, the communication node #1 serving as the source node of theworking path A and the protection path A becomes unavailable to receivethe BDI-ODUk alarm through the working path A. In this case, thecommunication node #1 releases the physical connection setting on theprotection path A, performs the physical connection setting on theworking path A, and transmits the user traffic A to the working path A.Meanwhile, the communication node #1 transmits the OCI-ODUk alarm to theprotection path A (see FIG. 31). Here, the ODUk frame of the usertraffic A transmitted from the communication node #1 to the protectionpath A includes, for example, the link ID of the link A.

Meanwhile, since the reception of the BDI-ODUk alarm through the workingpath B is detected in the communication node #8 serving as the sourcenode of the working path B and the protection path B, the communicationnode #8 maintains the physical connection setting on the protection pathB and transmits the user traffic B to the protection path B (see FIG.32). Here, the ODUk frame of the user traffic B transmitted from thecommunication node #8 to the protection path B includes, for example,the link ID of the link B.

The communication node #5 serving as the source node of the common pathC detects the reception of the OCI-ODUk alarm through the protectionpath A and receives the user traffic B through the protection path B(the OCI-ODUk alarm has been canceled). Thus, the communication node #5selects by using the selector 312 the user traffic B to be transferred,which is received through the protection path B (see FIG. 33). Thereby,the user traffic B is transferred to the communication node #7 throughthe communication node #6.

The end node #7 of the common path C copies the user traffic B receivedfrom the communication node #6 using the splitter 311 and transmits theuser traffic B to the respective communication nodes #4 and #9 servingas the end node of the working paths A and B (see FIG. 34).

The end node #4 of the working path A receives the user traffic Aincluding the link ID of the link A in the ODUk frame from the workingpath A. Meanwhile, the communication node #4 receives the user traffic Bincluding the link ID of the link B in the ODUk frame from theprotection path A (from the end node #7 of the common path C). Thecommunication node #4 selects, for example, the working path A (the usertraffic A) to be transferred based on the link ID using the selector 312(see FIG. 35).

Meanwhile, the communication node #9 serving as the end node of theworking path B receives the AIS-ODUk alarm from the working path B andreceives the user traffic B including the link ID of the link B in theODUk frame from the protection path B (from the end node #7 of thecommon path C). The communication node #9 selects by using the selector312 the user traffic B received from the protection path B (thecommunication node #7) through which the AIS-ODUk alarm has not beenreceived (see FIG. 36).

As described above, when the failure of the working path A is restoredin the state in which the failures occurred in both of the working pathsA and B, the protection path A can be switched back to the working pathA, and the user traffic B of the working path B can be restored by theprotection path B.

In the above-described example, when the failure of the working path Ais cleared, the protection path A is switched back to the working path Abut the protection path A is allowed not to be switched back to theworking path A. However, the user traffic B of the working path B cannotbe restored by the protection path B. Thus, it is preferable to switchthe protection path A back to the working path A.

The operation according to the above-described embodiment may besummarized as set out below. The source nodes #1 and #8 of the workingpaths A and B allocate a link ID to the ODUk frame and transfer the usertraffics A and B to the downstream communication nodes #2 and #10,respectively. Meanwhile, the source nodes #1 and #8 transmit theOCI-ODUk alarm to both of the protection paths A and B.

When the failure occurs in the working path A or B and the AIS-ODUkalarm or the BDI-ODUk alarm is detected, the source node #1 (or #8)switches the working path A (or the working path B) to the protectionpath A (or the protection path B).

The source node #5 which shares the protection paths A and B selects theODUk frame to be transferred to the downstream communication node #6(the common path C) in response to the detection of the OCI-ODUk alarm,the cancellation of the OCI-ODUk alarm, or the priority of the path.

The source node #7 which shares the protection paths A and B copies theODUk frame received from the upstream communication node #6 through thecommon path C and transfers the same ODUk frame to the respective endnodes #4 and #9 of the working paths A and B.

The end node #4 (or #9) of the working path A (or B) determines whetherthe link IDs of the ODUk frames received from both of the working path A(or B) and the communication node #7 indicate a transferable frame andselects the ODUk frame to be transferred according to the determinationresult.

As described above, according to the present embodiment, even when afailure occurs in all of the working paths sharing the protection path,path switching can be performed quickly without transmitting a controlsignal such as the APS signal.

(Others)

In the above embodiment, an OTN has been described as an example of acommunication network but the communication network may be, for example,any other network compatible to a synchronous digital hierarchy (SDH) ora synchronous optical network (SONET).

All examples and conditional language recited herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent inventions have been described in detail, it should beunderstood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A communication system, comprising: a source nodeand an end node of a first working path; a source node and an end nodeof a second working path; and a source node and an end node of a commonpath serving as a protection path shared by the first and second workingpaths, wherein the source node of the common path transfers one ofsignals received from the source nodes of the first and second workingpaths to the common path, and the end node of the common path transmitsthe signal received through the common path to each of the end nodes ofthe first and second working paths.
 2. The communication systemaccording to claim 1, wherein each of the source nodes of the first andsecond working paths transmits a dummy signal to the common path when afailure does not occur in the first and second working paths, and when afailure occurs in either or both of the first and second working paths,the source node of the failure occurred working path transmits a clientsignal to be transmitted to the failure occurred working path to thesource node of the common path.
 3. The communication system according toclaim 2, wherein, upon receiving the client signal from one of thesource nodes of the first and second working paths and the dummy signalfrom the other of the source nodes, the source node of the common pathtransfers the client signal to the common path.
 4. The communicationsystem according to claim 2, wherein each of the source nodes of thefirst and second working paths allocates link identification informationrelated to the working path to the client signal, and each of the endnodes of the first and second working paths selects a signal to betransferred based on the link identification information allocated tothe received client signal.
 5. The communication system according toclaim 3, wherein each of the source nodes of the first and secondworking paths allocates link identification information related to theworking path to the client signal, and each of the end nodes of thefirst and second working paths selects a signal to be transferred basedon the link identification information allocated to the received clientsignal.
 6. The communication system according to claim 2, wherein, inresponse to a recover of the failure occurred working path, the sourcenode of the recovered working path transmits the client signal havingbeen transferred to the source node of the common path to the recoveredworking path and transmits the dummy signal to the common path.
 7. Thecommunication system according to claim 3, wherein, in response to arecover of the failure occurred working path, the source node of therecovered working path transmits the client signal having beentransferred to the source node of the common path to the recoveredworking path and transmits the dummy signal to the common path.
 8. Thecommunication system according to claim 4, wherein in response to arecover of the failure occurred working path, the source node of therecovered working path transmits the client signal having beentransferred to the source node of the common path to the recoveredworking path and transmits the dummy signal to the common path.
 9. Thecommunication system according to claim 5, wherein, in response to arecover of the failure occurred working path, the source node of therecovered working path transmits the client signal having beentransferred to the source node of the common path to the recoveredworking path and transmits the dummy signal to the common path.
 10. Acommunication control method in a communication system including asource node and an end node of a first working path, a source node andan end node of a second working path, and a source node and an end nodeof a common path serving as a protection path shared by the first andsecond working paths, the communication control method comprising:transferring, from the source node of the common path, one of signalsreceived from the source nodes of the first and second working paths tothe common path; and transmitting, from the end node of the common path,the signal received through the common path to each of the end nodes ofthe first and second working paths.
 11. A transmission apparatus,comprising: a first receiver configured to receive a first signal from afirst node, the first node transmitting a signal to a first workingpath; a second receiver configured to receive a second signal from asecond node, the second node transmitting a signal to a second workingpath; and a signal transfer unit configured to transfer one of the firstsignal and the second signal to a common path serving as a protectionpath shared by the first working path and the second working path. 12.The transmission apparatus according to claim 11, wherein the firstreceiver receives a client signal to be transmitted through the firstworking path from the first node in response to an occurrence of failurein the first working path, and the signal transfer unit transfers theclient signal to the common path upon receiving the client signal andthe second signal.
 13. A transmission apparatus, comprising: a receiverconfigured to receive a signal transmitted through a common path servingas a protection path shared by a first working path and a second workingpath; and a transmitter configured to transmit the received signal to afirst node and a second node, the first node receiving a signaltransmitted through the first working path and the second node receivinga signal transmitted through the second working path.
 14. A transmissionapparatus, comprising: a first transmitter configured to transmit aclient signal to a working path; a second transmitter configured totransmit a dummy signal to a common path serving as a protection pathshared by the working path and another working path; and a failuredetector configured to detect a failure of the working path, wherein thesecond transmitter transmits the client signal to the common path inresponse to a detection of the failure.
 15. The transmission apparatusaccording to claim 14, further comprising, a link identificationinformation allocator configured to allocate link identificationinformation related to the working path to the client signal.
 16. Atransmission apparatus, comprising: a first receiver configured toreceive a first signal transmitted through a working path; a secondreceiver configured to receive a second signal transmitted through acommon path serving as a protection path shared by a first working pathand a second working path; and a signal transfer unit configured totransfer one of the first signal and the second signal, wherein thesecond receiver receives a signal from the common path, the signal beingsame as a signal transmitted to another node, the node receiving asignal transmitted through the second working path.
 17. The transmissionapparatus according to claim 16, wherein the signal transfer unitselects the signal to be transferred based on link identificationinformation related to the working path and allocated to the receivedsignal.